This invention relates to ultrasonic imaging systems and methods which utilize ultrasound echo information at a harmonic of the fundamental frequency of transmitted ultrasonic energy either for image formation or for aberration correction value estimation.
In an ultrasound imaging system, the velocity of sound is usually assumed constant in tissue in order to calculate time delays in forming acoustic beams from transducer arrays. However, the velocity of ultrasound waves in body tissues varies over a wide range. Therefore, ultrasound waves experience wavefront distortion, which disrupts diffraction patterns and produces image artifacts.
Several approaches have been proposed to correct for sound velocity inhomogeneities in tissue. One approach is to model the sound velocity inhomogeneities as a simple phase screen at or near the face of the transducer. Under this condition, sound velocity inhomogeneities result in time-of-flight errors (i.e., phase aberrations) and the received signal in one channel can be approximated by a time-delayed replica of the signal received by another channel. Therefore, phase aberrations can be estimated (1) by determining the peak position in the cross-correlation of signals received by two adjacent channels or subarrays (S. W. Flax and M. O'Donnell, "Phase aberration correction using signals from point reflectors and diffuse scatterers: basic principles," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 35, no. 6, pp. 758-767, 1988), or (2) by maximizing speckle brightness via time delay adjustment (L. F. Nock, G. E. Trahey, and S. W. Smith, "Phase aberration correction in medical ultrasound using speckle brightness as a quality factor," J. Acoust. Soc. Am., vol. 85, no. 5, pp. 1819-1833). Another proposed method estimates aberrating delays by utilizing array redundancy in spatial frequency (D. Rachlin, "Direct estimation of aberrating delays in pulse-echo imaging systems," J. Acoust. Soc. Am. vol. 88, no. 1, pp. 191-198, 1990).
The validity of the near field thin phase screen model has been questioned, based on the fact that waveform distortions in addition to time delay errors have been observed. (D. L. Liu and R. C. Waag, "Correction of ultrasonic wavefront distortion using backpropagation and a reference waveform method for time shift compensation," J. Acoust. Soc, Am., vol. 96, no. 2, pp. 649-660, 1994). These waveform distortions have been explained by modeling the acoustic velocity inhomogeneities as distributed throughout the region between the transducer and the target or by putting the phase screen at a distance away from the face of the transducer. Various methods have been proposed to correct for distributed aberrations (or displaced phase screens). They include a back propagation method (Liu, et al., supra), a total least squares (TLS) based approach called PARCA (S. Krishnan, P. C. Li, and M. O'Donnell, "Adaptive compensation of phase and magnitude aberrations," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 43, no. 1, pp. 44-55, 1996), and a time reversal focusing technique (M. Fink, "Time reversal focusing in ultrasound: basic principles," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 39, no. 5, pp. 555-566, 1992).
Recently, other alternative approaches have also been developed to correct for distributed aberrations. They include a phase conjugation approach (G. C. Ng, P. D. Freiburger, W. F. Walker, and G. E. Trahey, "A speckle target adaptive imaging technique in the presence of distributed aberrations," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 44, no. 1, pp. 140-151, 1997), which independently corrects for time delay errors for each frequency component, and an inverse filtering approach (Q. Zhu and B. Steinberg, "Deaberration of incoherent wavefront distortion: an approach toward inverse filtering," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 44, no. 3, pp. 575-589, 1997), which compensates for both phase and amplitude distortion in the frequency domain.
One major factor that determines the efficacy of all the methods mentioned above is the quality of the transmit beam profile. A good transmit beam profile (narrow mainlobe and low sidelobes) improves both the image quality and the estimation accuracy. It has been shown that harmonically generated transmit beam profiles have lower sidelobes and are less sensitive to the phase aberrations that are present. (T. Christopher, "Finite amplitude distortion-based inhomogeneous pulse echo ultrasonic imaging," IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 44, no. 1, pp. 125-139, 1997).
The above-referenced Christopher article speculates as to aberration correction in a harmonic ultrasonic imaging system, but provides no details as to the structure or operation of any such system.
Wright U.S. Pat. No. 5,570,691, assigned to the assignee of the present invention, discloses one particularly advantageous aberration correction value estimation system in which ultrasonic energy from a single firing or transmit event is used both in the formation of the ultrasonic image and in the calculation of aberration correction values. In this way, the need for separate aberration correction lines or frames can be eliminated.
Johnson U.S. Pat. No. 5,456,257 discloses systems for ultrasonically detecting contrast agents. In one disclosed embodiment signals from collapsing microbubbles included in a contrast agent are used to calculate delay adjustments intended to correct for tissue aberration. Little detail is provided regarding the structure of the disclosed system, and contrast agent is essential for operation of the disclosed system.